Sub-Saharan Africa is one of the hardest-hit areas, where ecological degradation, poor water management and a burgeoning population have led to water shortages exacerbating poverty, disease and drought, the report said.

The report was compiled by 24 U.N. agencies, who say it is the most comprehensive assessment to date of the planet's freshwater supplies.

Globally, diarrheal diseases and malaria kill about 3.1 million people a year. The U.N. said 1.6 million could be saved if they had safe drinking water, sanitation and hygiene.

The report estimated that hundreds of millions of dollars in lost productivity and health care costs are lost each year because of poor water and sanitation. Meeting the U.N. Millennium Development Goal of halving the number of people without a steady supply of clean water by 2015 would save $7 billion annually, the report said.